The invention is based on a process for the utilization of municipal or industrial sewage sludges with high water contents by briquetting as preparation for a subsequent thermal or chemical processing.
Sewage sludges from municipal or industrial sewage farms arise in large tonnages annually and are disposed of at present by dumping, use in agriculture and also combustion in special sewage sludge incineration plants. Legislation is continually setting more stringent requirements for these methods of disposal, so that e.g. in future the disposal of sewage sludges in agriculture will be permitted only if their mercury content does not exceed a certain upper limit and the mercury content of the soil does not exceed certain limits. Furthermore, the deposition will be very greatly restricted in future by the German Technical Directive on Municipal Waste, which is coming into effect, and made practically impossible by the limitation of the organic component of the deposited product to a maximum of 5%. Because of that, apart from the localized use in agriculture of sludges with fairly low heavy metal contamination, only incineration or chemical-thermal utilization are still available as methods of disposal.
Of these methods, the development of the technique of the monocombustion of sewage sludge is the furthest advanced, but it has the disadvantage that, owing to the usually low calorific values of the sewage sludge, considerable amounts of high-grade fuels such as gas or oil must be fired in addition in order to reach the required combustion temperature. The calorific value of these unavoidably required fuels is also usually utilized within the sludge incineration plants only at a low energy level for the generation of low-pressure steam. The actual combustion is often preceded by a drying stage, which additionally requires energy. Moreover, incineration plants for sewage sludge are very intensive in investment costs. A better possibility, from the viewpoint of energy and of investment costs, for the thermal utilization of sewage sludges consists in the co-combustion in existing power station plants, above all in slag tap fired boilers, wherein at the same time the energy content of the sewage sludge can be utilized at a high energy level. However, this form of utilization makes great demands on the preparation of the sewage sludge, e.g. it must be available either in briquetted or in dry granulated form, the granulation and briquetting always requiring, according to the current state of the art, an upstream drying stage which is intensive in investment and operating costs (DE 4 013 206). The briquetting of sewage sludges, on the other hand, is regarded according to the state of the art and prevailing expert opinion as feasible only for dried sludges below a water content of about 20% (DE 3 606 704). A process is proposed in DE 3 243 827 for briquetting sewage sludges with dried autumn leaves or their extraction residues. This process is unusable for a large-scale industrial process for logistical reasons and because of the tying of the raw material preparation to a particular season.
In DE 3 010 259, a process for the production of briquettes from organic wastes is described.
The disadvantages of the process include the necessity of adding CaO to the briquette composition. An additional substance must therefore be prepared and the calorific value of the briquettes is reduced by contamination of the fuel by inerts. In addition, considerable risks to the boilers occur as a result of Ca deposits and corrosion. A further disadvantage of the process consists in the necessity of applying very high pressures, which causes on the one hand high investment in machinery and on the other hand high operating costs. A further disadvantage of the process, reducing its productivity, consists in the long period of action of the pressure on the material. Further disadvantages consist in the restricted applicability of the process for wastes of all kinds, since these behave quite differently during the moulding. Thus the moulding behaviour of plastics, for example, is not comparable with that of domestic refuse.
The problem therefore arises of discovering a method for the processing of the sewage sludge to a supplementary fuel of low calorific value for further chemical or thermal utilization, which requires only small investment in the downstream utilization plant, which usually already exists for other purposes, puts the sewage sludge into a "suitable form for a power station", enables the sewage sludge to be reliably compacted under all conditions, at the same time avoids an expensive drying of the sewage sludge, as well as permitting a simple logistical handling of the sewage sludge, and dispensing with particular additives and the application of high pressures.